1. Field of the Invention
The invention relates to high performance polymers. In particular, this invention relates to polyimide polymers, which have many desirable properties, such as thermal stability and strength.
2. Description of the Related Art
Polyimides are an important class of polymeric materials and are known for their superior performance characteristics. These characteristics include high glass transition temperatures, good mechanical strength, high Young's modulus, good UV durability, and excellent thermal stability. Most polyimides are comprised of relatively rigid molecular structures such as aromatic/cyclic moieties.
As a result of their favorable characteristics, polyimide compositions have become widely used many industries, including the aerospace industry, the electronics industry and the telecommunications industry. In the electronics industry, polyimide compositions are used in applications such as forming protective and stress buffer coatings for semiconductors, dielectric layers for multilayer integrated circuits and multi-chip modules, high temperature solder masks, bonding layers for multilayer circuits, final passivating coatings on electronic devices, and the like. In addition, polyimide compositions may form dielectric films in electrical and electronic devices such as motors, capacitors, semiconductors, printed circuit boards and other packaging structures. Polyimide compositions may also serve as an interlayer dielectric in both semiconductors and thin film multichip modules. The low dielectric constant, low stress, high modulus, and inherent ductility of polyimide compositions make them well suited for these multiple layer applications. Other uses for polyimide compositions include alignment and/or dielectric layers for displays, and as a structural layer in micromachining applications.
In the aerospace industry, polyimide compositions are used for optical applications as membrane reflectors and the like. In application, a polyimide composition is secured by a metal (often aluminum, copper, or stainless steel) or composite (often graphite/epoxy or fiberglass) mounting ring that secures the border of the polyimide compositions. Such optical applications may be used in space, where the polyimide compositions and the mounting ring are subject to repeated and drastic heating and cooling cycles in orbit as the structure is exposed to alternating periods of sunlight and shade.
Polyimide polymers are subject to rapid degradation in a highly oxidizing environment, such as an oxygen plasma or atomic oxygen [AO], as are most hydrocarbon- and halocarbon-based polymers. AO is present in low earth orbit [LEO], so many spacecraft experience this highly oxidizing environment. The interactions between the oxidizing environment and the polymer material can erode and reduce the thickness of the polymer material. To prevent the erosion, protective coatings including metals, metal oxides, ceramics, glasses, and other inorganic materials are often applied as surface treatments to polyimides subjected to the oxidizing environment.
While these coatings are effective at preventing the oxidative degradation of the underlying material, they often experience cracking from thermal and mechanical stresses, mechanical abrasion, and debris impact. After cracking, the protective surface is compromised and the underlying polymeric material can be degraded from additional exposure to the oxidizing environment. Therefore, the availability of polymers which are able to resist AO degradation is very desirable.
Oligomeric silsesquioxanes [OS] can be incorporated into a polyimide matrix to improve the durability of polyimides in these environments. Polyimides with incorporated OS demonstrate excellent resistance to AO degradation prevalent in LEO environments. Polyimides with incorporated OS provide additional benefits as well. Polyhedral OS are referred to by the trademark POSS™, and are a common form of OS. There are examples of polyimide polymers which have incorporated OS currently in existence.
The article by Leu et al., “Synthesis and Dielectric Properties of Polyimide-Tethered Polyhedral Oligomeric Silsesquioxane (POSS) Nanocomposites via POSS-diamine” (Macromolecules 2003, 36, 9122-9127 (2003)) describes a polyimide polymer having a polyhedral OS group attached to the polymer backbone with an organic tether. The polyhedral OS is incorporated into the polymer by attachment to a diamine monomer. The diamine monomer requires three reaction and purification steps to synthesize and purify, and an additional reaction step to incorporate the monomer into the polyimide polymer with pyromellitic dianhydride (PMDA) utilized as the dianhydride monomer.
Wright et al. discloses a polyhedral OS containing polyimide polymer in the article “Chemical Modification of Fluorinated Polyimides: New Thermally Curing Hybrid Polymers with POSS” (Macromolecules 2006, 39, 4710-4718 (2006)). The polyhedral OS is connected to the polymer backbone with an organic tether, and it is connected through an available alcohol group on the diamine monomer 3,5-bis(4-aminophenoxy)-1-hydroxymethylbenzene (BNB). 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6-FDA) is used as the dianhydride monomer, and the resulting polyimide polymer is soluble in certain organic solvents. The method described for creating the final polymer included isolating the polyimide polymer three times: 1) after the polyimide polymer was formed; 2) after the available alcohol group was modified to provide an acid chloride; and 3) after the polyhedral OS was attached to the acid chloride. Additionally, the BHB monomer was prepared in the lab, because this compound is not commercially available.
Lichtenhan et al., in U.S. Pat. No. 6,933,345, describes many polymers including polyhedral OS groups. Lichtenhan describes the polyhedral OS being blended with the polymer, being reacted into the polymer backbone, or being tethered to the polymer backbone. The specific characteristics of the polymer are not disclosed, and no method for producing a polyimide polymer with a polyhedral OS group attached through an organic tether is disclosed.
US Patent Application 2006/0122350 by Wei et al. discloses a polyimide polymer with a polyhedral OS group attached to the polymer backbone with an organic tether. The polyhedral OS group is attached through either an alkyl carbon or through a benzene ring connected into the polymer backbone at the 1st and 4th carbon. If the polyhedral OS group is attached to the polymer backbone at a benzene ring, that benzene ring does not connect directly to two imide groups. The method for preparing the polymer with the polyhedral OS group includes forming a monomer with the polyhedral OS group attached, and then making the polymer, or the polyimide was created and chlorinated POSS was reacted with the polyimide. The described process includes 2 isolations and purifications of the polyimide polymer.
Svejda et al., in U.S. Pat. No. 6,767,930 describes incorporation of polyhedral OS groups in polymers in general. The polyhedral OS is incorporated by non-reactive blending, reactive grafting, reactive polymerization into the polymer backbone, and reactive cross-linking. The specific use of polyhedral OS groups in polyimide polymers includes both non-reactively blending the polyhedral OS group with a polyimide polymer, and reactively polymerizing the polyhedral OS with the polyimide such that the polyhedral OS group forms part of the polymer backbone.
Polyimide polymers have many desirable characteristics. Some characteristics of polyimide polymers can be improved by the incorporation of OS.